The invention is related to a process for producing an absorbent polymer structure, an absorbent polymer structure obtainable by this process, an absorbent polymer structure, a compound, a process for producing a compound, a compound obtainable by this process, chemical products comprising the absorbent polymer structure or the compound, the use of the absorbent polymer structure or the compound in chemical products, an aqueous solution, a process for producing the aqueous solution, an aqueous solution obtainable by the process as well as the use of the aqueous solution in treatment of the outer portion of an absorbent polymer structure.
Superabsorbers are water insoluble cross-linked polymers, which are capable, by swelling and formation of hydrogels, of absorbing large quantities of aqueous liquids, in particular body liquids, preferably urine or blood, and retaining them under a certain pressure. Because of these characteristic properties these polymers find applications mainly through incorporation into sanitary articles, such as, for example, baby diapers, incontinence products or sanitary napkins.
Existing commercially available superabsorbers are essentially made up of cross-linked polyacrylic acids or cross-linked starch-acrylic acid graft polymerizates, in which the carboxyl groups are partly neutralized with sodium hydroxide or potassium hydroxide.
For aesthetic reasons and on environmental grounds there is an increasing tendency to create ever smaller and thinner sanitary articles. In order to ensure a consistent overall retention capacity of the sanitary article, this demand can only be met through reduction of the proportion of voluminous fluff. This means that further tasks fall to the superabsorber with respect to transport and distribution of liquids, which can be summarised as permeability properties.
By permeability in superabsorbent materials is understood the ability, in the swollen state, to transport and distribute added liquids in three dimensions. This process proceeds in the swollen superabsorbent gel via capillary transport through gaps between the gel particles. Liquid transport through swollen superabsorber particles themselves follows the laws of diffusion and is a very slow process, which plays no role in the distribution of the liquid in the utilization situation of the sanitary article. For superabsorbent materials that cannot accomplish capillary transport for reasons of lack of gel stability, a separation of the particles from each other to prevent the gel-blocking phenomenon was ensured by embedding these materials in a fiber matrix. In newer generations of diaper constructions the absorbent layer contains only little or indeed no fibrous material at all to support liquid transport. Superabsorbers used here must thus possess a sufficiently high stability in the swollen state such that the swollen gel still contains a sufficient quantity of capillary spaces, through which the liquid can be transported.
In order to obtain superabsorbent materials with higher gel stability, on the one hand the degree of cross-linking of the polymer can be increased, which inevitably results in a decrease of the swelling ability and the retention capacity. An optimized combination of different cross-linkers and co-monomers, as described in DE 196 46 484, does enable the improvement of the permeability properties, but not to a level which permits for example the fitting of a layer composed optionally of only superabsorbers into a diaper construction.
Furthermore, methods for secondary treatment of the surface of polymer particles to improve the superabsorbent properties can be applied. As surface treatment are known from prior art for example secondary cross-linking of the absorbent polymer structure at the surface, the bringing into contact of the surface with inorganic compounds or the secondary cross-linking of the surface in the presence of inorganic compounds.
Thus EP-A-0 450 923, EP-A-0 450 922, DE-A-35 23 617, U.S. Pat. No. 5,140,076 and U.S. Pat. No. 4,734,478 describe the treatment of the surface of absorbent polymers by bringing the surface into contact with inorganic compounds, such as for example dispersed silica, during or after the secondary cross-linking of the surface. Next to an increased rate of absorption under pressure an increased permeability of the absorbent polymer is also achieved by this type of surface treatment.
DE 35 03 458 describes a process for producing an improved absorbent resin, wherein a water absorbent resin, which contains units of a monomer, said monomer having a carboxyl group in the form of the free acid or of a salt, as a constituent component of the resin, in the presence of a powder of a finely divided metal oxide, permits the absorption of a cross-linking medium and water and the resulting mixture is heated while agitating, in order to effect the cross-linking of the resin and the removal of the water. Absorbent resins with a good water absorption capacity are obtained hereby, which at the same time have a good rate of absorption.
U.S. Pat. No. 4,535,098 describes a process for increasing the gel strength of non-secondary cross-linked superabsorbers by swelling of absorbent polymers in the presence of a dispersed colloidal inorganic compound, such as a silica sol, or by producing an absorbent polymer in the presence of a dispersed colloidal inorganic compound.
DE 198 05 447 discloses a process for secondary cross-linking of polyacrylonitrile hydrolysates with bifunctional compounds and a simultaneous immobilization of silica in the surface structure of superabsorbent polymers. The silica together with the cross-linking medium in a water/alcohol mixture was brought into contact with the surface. By immobilizing the silica an improvement of the Absorbency under Load as well as a decrease in gel blocking should be achieved.
DE 198 54 575 describes the addition of alkali salts of silicic acid before, during or after the polymerization or in partial neutralization of the superabsorber. Through this surface treatment an improved permeability is achieved, which is however principally ascribed to the partially reduced retention of the polymers caused by non-swellable additive.
U.S. Pat. No. 5,147,921 discloses the addition of a silica sol as inert filler, which can be dispersed in the monomer solution to be polymerized.
JP 1994-16822 describes the secondary treatment of the surface of absorbent polymers with an inorganic sol. In the context of the processability of the mixture, which tends to form agglomerates, an additional organic solvent component is added. Mono- and dimethylethers of diols or diols themselves are cited as examples of organic solvent components. After drying the absorbent polymers should have higher gel stability, a reduced tendency to gel blocking and an improved permeability to water in simple tests without pressure stress on the superabsorber.
The prior art describes processes by which inorganic particles are either dry blended with the superabsorber or introduced into the secondary cross-linking process with the aid of a large quantity of in part organic solvent, in order to prevent an agglomeration of the superabsorber particles. These processes present however the disadvantage, that either large quantities of solvents must be handled, which is as undesirable on economic as it is on ecological grounds. Furthermore superabsorbent polymers tend to agglomerate when mixed with large quantities of liquids, which can seriously impair the processability within a continual production process. A simple blend with inorganic finely divided substances on the other hand leads to disadvantages, such as decomposition or dust. The addition of inorganic additives in aqueous solutions to the cross-linking itself is difficult, since the inorganic particles are quickly deposited. Moreover, it can be difficult to dose accurately inorganic dispersions.
Partly through the presence of the finely divided inorganic substances disclosed in the prior art, an inhomogeneous distribution of the chemical secondary cross-linkers occurs on the surface of the absorbent polymers and accordingly also an inhomogeneous cross-linking. This in turn leads to superabsorbent polymers being obtained with a dissatisfactory overall performance, above all with respect to retention and permeability. A homogeneous distribution in the surface treatment processes in the prior art is in all cases possible by using large quantities of an aqueous or alcoholic solution containing the chemical cross-linker.
The general objects underlying the present invention are to overcome the drawbacks arising from the state of the art.
Furthermore, an object according to the invention consists in making available superabsorbent polymers which unite as a combination of properties not only a high absorption capacity under pressure but also the typically opposing properties of a high retention capability and a good permeability, in order to satisfy the demands on absorbent polymers of modern hygiene articles, in particular diapers, incontinence products or sanitary napkins. In particular these polymers should comprise the smallest possible quantities of toxic monomers, such as acrylamide or acrylonitrile, which are washed out upon a contact of the superabsorbent polymer with body liquids and, for example in the case of the use of the superabsorbent polymers in diapers, could in this way come into contact with the skin of the person wearing the diaper.
A further object underlying the present invention consisted in the provision of hygiene articles such as for example diapers, which in comparison to the hygiene articles known from the state of the art are better able to retain absorbed body liquids, to absorb liquids under pressure, and, on absorbing liquids, to distribute these liquids as quickly and evenly as possible in the hygiene article.
In addition, another object according to the invention lies in the creation of a process with which such absorbent polymers can be produced with the smallest possible quantities of organic solvents. In this production process added inorganic additives should displace superabsorbent polymer at most in small amounts, which do not negatively influence the polymer properties. The solution employed in this process for treatment of the surface of the absorbent polymer should be able to be handled as a single-phase system and be consistently dosed. The coated superabsorber should form aggregates to only a slight extent in the course of the process and should be able to be applied simply to a continuously operating tempering step.